Lysozyme is a cationic protein of 146 amino acid residues (Mr approximately 14k) that is bactericidal against multiple gram-positive bacteria in vitro. Although lysozyme is the most abundant antimicrobial protein in airway surface fluid (ASL), its role in protecting the airways against infection, chronic colonization, and inflammation in vivo remains unproven. The results of preliminary studies reported in this application demonstrate a dose-dependent relationship between the level of lysozyme activity in bronchoalveolar lavage fluid and the rate of bacterial killing, of both gram positive and negative organisms, in acutely infected transgenic mice that overexpress lysozyme. Studies proposed in this application will test the central hypothesis that lysozyme is a critical component of airway host defense in vivo. Pathogen killing will be assessed in lysozyme (-/-) mice and lysozyme overexpressing mice following acute and chronic lung infection. The antimicrobial spectrum and potency of exogenously-administered recombinant lysozyme protein will also be assessed. These studies will provide insight into the role of lysozyme in airway host defense and provide a preliminary assessment of the therapeutic potential of exogenously administered lysozyme.